The present invention provides further improvements in fabricating plastic pieces such as lids for food containers that is disclosed in U.S. Ser. No. 09/249,401 entitled "Portable Food Container Cover With Detachable Utensil" (filed Feb. 12, 1999 and which is incorporated herein by reference, attorney docket 5202). The patent application describes improved plastic cover lids for food containers that include a detachable two piece utensil.
The covers are taught as being useful for food containers such as for refrigerated yogurt. In the fabrication of these plastic pieces, plastic resin is melted to form a hot melt plastic that is fed into a blow molding or injection molding device. In the mold, the hot plastic is formed into desired shape and is quickly cooled to about room temperature to form solid pieces, that are then ejected or otherwise removed from the injection molding device.
Frequently, it is desirable to apply a label to formed plastic pieces. For example, the described two piece utensil includes a protective membrane overlaying the cover to keep the utensil clean. In other applications, a food label may be desired to adhere, for example, to the side wall of a bottle or to an open end as a tamper evidence seal.
One problem with applying a label to a molded plastic piece is that such pieces continue to shrink modestly even when kept at room temperature. While pieces continue to shrink over time, most shrinkage occurs during the first 48 hours after molding. It is speculated that this shrinkage is due to gradual release of stress locked into the pieces during formation. This shrinkage over time is sometimes referred to in the art as "aging".
Although the shrinkage is minute, if a label is applied before substantial completion of the shrinkage, then the label may be rendered defective. For example, the label may appear wrinkled or may rupture. For labels that are intended to seal the container, stretch gaps can develop leading to the loss of seal integrity. Premature product spoilage can occur. As a result, in-line labeling (i.e., labeling shortly after lid fabrication) in many applications heretofore has not been practical.
In the past, such shrinkage has been considered in the art to be a natural and unavoidable problem. One possible solution to shrinkage problems is to warehouse the pieces for a period of time until acceptable dimensional stability occurs. However, such an approach especially in mass production requires expensive inventory, space and containers to house the inventory and further requires expensive additional handling. Also, product contamination is more likely with warehousing and increased handling.
A wide variety of approaches tried by the inventors fail to solve the problem of shrinkage. For example, various modifications to the film composition for the label have been tried unsuccessfully. Alternate adhesives for the label have likewise failed. Various fillers have been added to the plastic pieces to attempt to arrest the shrinkage have been unsuccessful.
After many failed attempts, surprisingly, the present invention provides a solution for the problem of gradual shrinkage over extended times in plastic pieces.
In one important aspect, the present invention resides in thermal annealing or temperature aging of the plastic pieces to accelerate expected shrinkage to provide substantially dimensionally stable plastic pieces suitable for immediate labeling.
The present invention provides the important advantage of in-line labeling. As a result, a continuous fabrication process is possible including applying a label within minutes of piece forming.
A further advantage is that the risks and expensive inventory and warehousing are substantially reduced.
Reductions are also provided in the type number and degree of defective labels.
A further advantage resides in increases in rates of production permitted by the present invention.
Still another advantage is that plastic pieces that are relatively more dimensionally stable can be provided immediately after fabrication for uses other than for labeling.
Still another surprising advantage resides in the simplicity of the present improvements.
Additional features and advantages of the present invention are described in, and will be apparent from, the detailed description of the presently preferred embodiments taken in conjunction with the Drawings, in which: